1. Field of the Invention
The present invention relates to an image pickup apparatus capable of capturing a composite image of a visible-light image and an infrared image from an infrared fluorescent substance.
2. Description of the Related Art
Phosphors are utilized in techniques for detecting the structure of an object that is not apparent in a visible-light image, or for detecting the presence of a specific substance within an object. For example, the microscope instrumentation disclosed in Japanese Laid-open Patent Application No. 10-325798 is used to observe a biological tissue sample in which an antibody bonded with a phosphor referred to as indocyanine green (ICG hereinafter) is used as a probe. This microscope instrumentation detects the presence of minute cancers in the biological tissue sample through the observation of phosphorescence generated from the probe, which has the property of easily accumulating in the cancer tissue.
ICG is a dye that absorbs and is excited by near-infrared light having a wavelength of approximately 800 nm, and fluoresces to emit near-infrared light of approximately 850 nm. ICG can be administered to the human body as a contrast medium, and the infrared light images obtained through the use of ICG are used to diagnose and treat illnesses. For example, an image of a blood vessel can be observed by injecting ICG into the blood vessel. Liver functioning is tested using the property of ICG whereby ICG binds to proteins in the blood, and is selectively taken into the livers.
In surgery and the like, the physician must be able to visually monitor the operating site. During surgery, it is sometimes preferable for an image to be obtained in which the position of a blood vessel on or under the surface of the operating site can be monitored, in addition to the usual image obtained as a visible-light image. From this perspective, there is a need for an image (infrared composite image) in which an infrared image obtained through the use of a fluorescent contrast medium is synthesized with a visible-light image.
An image pickup apparatus capable of capturing a visible-light image and an infrared image is necessary in order to generate an infrared composite image. A CCD image sensor or other solid-state image sensor is sensitive not only to visible light, but also to near-infrared light. Therefore, a solid-state image sensor may be used to capture the infrared composite image.
In this regard, in the abovementioned microscope instrumentation, a dichroic mirror is used that reflects the excitation light of the ICG and transmits the fluorescence of the ICG to reflect the excitation light emitted from a light source towards the test sample, and the fluorescent component among the fluorescent component and the excitation light component included in the light from the sample is selectively transmitted and directed into the solid-state image sensor. In the microscope instrumentation, the dichroic mirror is set so as to reflect a portion of the visible light and transmit the rest so that the visible-light image is observed at the same time as the infrared image from the phosphor. In this arrangement, the reflectance and transmittance of visible light are each set to 50%, whereby, theoretically, the percentage of the visible light emitted from the light source that reaches the solid-state image sensor can be maximized. In this regard, the intensity of the visible light that reaches the solid-state image sensor from the sample in the maximized state is 25% of the intensity that could be obtained if the visible light from the light source were to be directly radiated to the sample without passing through the dichroic mirror, and the reflected light allowed to directly enter the solid-state image sensor.
The apparatus disclosed in Japanese Laid-open Patent Application No. 10-325798 is known as an apparatus for enabling simultaneous observation of a visible-light image and an infrared image, but this apparatus is a microscope instrumentation, and it is not always possible to apply this configuration to observation of an operating site during surgery, or to other imaging applications. For example, in surgery, it is sometimes necessary to maintain a relatively large space for surgical maneuvering around the surgical operating site, or to maintain a space for enabling the surgeon to directly see the operating site during surgery. In such cases, it can be difficult to maintain the necessary space for an image pickup apparatus that has constituent elements near the object being imaged. An image pickup apparatus used to observe a surgical site is preferably configured so that the arrangement of the imaging means or the illumination means has a high degree of freedom with respect to the observed object in order to enable imaging from an arbitrary position that does not obstruct the surgical operation.